1. Field of the Invention
The present invention generally relates to semiconductor fabrication and, more particularly, to the fabrication of a low-resistance multilayer structure and formation of ohmic contacts.
2. Description of Related Art
As semiconductor devices continue to scale down, the conventional polysilicon gate electrode is becoming inadequate. One problem is relatively high resistivity and another is depletion of dopants in the polysilicon gate electrode in proximity to the interface between the polysilicon gate electrode and the gate dielectric. In particular, as integration degree and speed of devices are increased, a signal delay arises from the resistances of the gate electrode and the interconnection. A “so-called” polycide gate including a metal silicide film and a polysilicon film has been proposed to reduce the resistance and in turn reduce the signal delay in the semiconductor devices such as a MOS transistor.
To overcome these deficiencies of polysilicon or polycide gate, poly-metal gates having a stack structure of a refractory metal film, a barrier metal film and a polysilicon film or metal gates are being pursued as an alternative. This is particularly true, if the gate electrode has a length of less than about 0.25 μm, requiring the use of a gate electrode having a resistance lower than that of the polycide gate.
More recently, among the poly-metal gates, a doped poly/tungsten (W) gate structure has received special attention as it has lower word line resistance and lower stack height. This is because tungsten has a low sheet resistance. For example, if a tungsten film is used as a refractory metal film, an RC signal delay time can be remarkably reduced because the resistivity of the tungsten is smaller than that of the tungsten silicide (WSix). Also, a dual poly/tungsten gate has become more popular as it has high current drivability and a lower threshold voltage.
However, the poly-metal gate structure is not without its limitations. For example, tungsten may react with polysilicon at high temperatures to form undesirable compounds, thereby increasing interface resistance therebetween and slowing down the operation speed of the semiconductor device. Some tungsten atoms may even diffuse into a gate dielectric layer, degrading the device characteristics. Thus, a barrier metal layer is formed between the tungsten and polysilicon to reduce or prevent any reaction between them.
Unfortunately, the interface resistance of such a barrier metal layer is known to be very high. Therefore, in the prior art, to reduce the interface resistance, an ohmic contact layer is formed between the metal barrier layer and the polysilicon layer. However, the ohmic contact layer, which is typically formed of a metal silicide, acts as a diffusion path of dopants, aggravating a so-called “poly depletion” problem. In particular, dopants such as boron (B) undesirably out-diffuse from a polysilicon gate during heat treatment processes such as rapid thermal processing (RTP), leading to a consequential low inversion capacitance of the transistor. Various approaches have been attempted to solve these problems.
Embodiments of the invention address these and other disadvantages of the conventional art.